336 CRUDE PETROLEUM or if very exact results are required, the picnometer. The determination should be made at 15° or referred to 15°, the mean temperature coefficient of specific gravity being 0-0007 (0-0006-0-0008) per i°. 2. Fractional Distillation.—In order that concordant results may be obtained, this should always be carried out under certain definite con- ditions and in an apparatus of fixed dimensions. Engler's flask, shown in Fig. 40, is generally used. 100 c.c. of the oil are placed in the flask which is connected with a condenser 60 cm. long and heated, both the flame and the flask being protected with a sheet metal mantle. The initial temperature of distillation is that at which the first drop of distillate issues from the condenser fitted to the side-tube of the flask. The velocity of distillation should be such that two drops pass over per second. The distillate is collected in several graduated cylinders, or in a single 100 c.c. cylinder in which the volumes of the distillates at the different FIG. 40 FIG. 41 temperatures are read off successively. The distillation is at an end when the flask contains only residuum or white fumes appear. The fractions usually collected are : Benzine: up to 150° C. Lamp oil: 150-300°. Heavy oils : above 300°. In the Italian Customs Laboratory, for the determination of the frac- tions distilling below 310° for fiscal purposes, use is made of a flask similar to the preceding but of the dimensions shown in Fig. 41. 100 c.c. of the petroleum are then distilled, the flask being heated over a gauze by a small flame ; subsequently the flame is enlarged, and it may finally be necessary to surround the flask with an asbestos mantle. The rate of dis- physical and partly chemical.